Polymeric materials exhibiting high CO2 sorption are attractive materials for their applicability as sorbent and gas separation membrane materials. Polybenzimidazole (PBI) based polymeric forms of ionic liquids (PFILs) have (i) excellent CO2 sorption characteristics (ii) improved solvent solubility than parent PBIs (needed for membrane preparation either in flat sheet form or hollow fiber form) and (iii) high enough thermal stability. PBI based polymeric forms of ionic liquids (PFILs) are not known in the literature. These materials since are derived from thermally stable PBIs and thus have high thermal stability than their PIL counterparts derived from vinyl monomers known in the prior art, which are aliphatic in nature.
US2002/0189444 claims a liquid ionic compound of formula given below
for separation of CO2 from natural gas.
WO2006/026064 describes a polymer comprising a polymerized ionic liquid monomer selected from (a) imidazolium-based ionic liquids, consisting of 1-[2-(methacryloyloxy)ethyl]-3-butylimidazolium tetrafluoroborate ([MABI][BF4]), 1-(p-vinylbenzyl)-3-butylimidazolium tetrafluoroborate ([VBBI][BF4]), (b) ammonium-based ionic liquids, consisting of (p-vinylbenzyl)trimethyl ammonium tetrafluoroborate ([VBTMA][BF4]), (p-vinylbenzyl)triethyl ammonium tetrafluoroborate ([VBTEA][BF4]), (p-vinylbenzyl)tributyl ammonium tetrafluoroborate ([VBTBA][BF4]), and (p-vinylbenzyl)trimethyl ammonium trifluoromethane sulfonamide ([VBTMA][Tf2N]), condensation polymerization ionic monomers consisting of bis(2-hydroxyethyl)dimethyl ammouium tetrafluoroborate ([BHEDMA][BF4]), 2,2-bis(methylimidazolium methyl)-1,3-propanediol tetrafluoroborate ([BMIMP][BF4]), and 2,2-bis(butylimidazolium methyl)-1,3-propanediol tetrafluoroborate ([BBIMP][BF4]) for absorption of CO2 gas.
U.S. Pat. No. 4,898,917 discloses a unique process for the preparation of N-substituted polybenzimidazole polymers from unsubstituted polybenzimidazole polymers. According to the process of the invention unsubstituted polybenzimidazole polymer is first reacted with an alkali hydride to produce a polybenzimidazole anion in DMSO which is then reacted with a substituted or an unsubstituted alkyl, aryl or alkenyl methyl halide at a temperature of about 50-120° C. for 5-48 hours to produce an N-substituted alkyl, alkenyl or aryl polybenzimidazole polymer. The substituted polybenzimidazole polymer produced by this process can be formed into membranes, films, resins or fibers. The polymers can be used in reverse osmosis, ultrafiltration, microfiltration, electrodialysis, ion exchange and affinity chromatography. It is further described that the composition of the R substituent depends upon the desired N-substituted polybenzimidazole end product.
An article titled “Advances in CO2 capture technology—The U.S. Department of Energy's Carbon Sequestration Program” by José D. Figueroa in INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL 2 (2008) 9-20, discusses the use of ionic liquid [hmim][Tf2N] for dissolution of CO2. The ionic liquid is shown to have good thermal stability allowing the recovery of CO2 without requiring it to cool. The article discusses the development in the use of PBI membranes in H2/CO2 selectivity. The article further mentions a supported liquid membrane that is CO2 selective and stable at temperatures exceeding 300° C. The membrane consists of an advanced polymer substrate and an ionic liquid. It is observed that in these supported liquid membranes transport takes place through the liquid within the pores rather than through a solid phase. This feature allows the membranes to take advantage of higher liquid phase diffusivities while maintaining the selectivity of the solution diffusion mechanism.
US2006/0021502 discloses cross-linked, supported polybenzimidazole membrane prepared by reacting polybenzimidazole (PBI) with the sulfone-containing crosslinking agent 3,4-dichloro-tetrahydro-thiophene-1,1-dioxide. The cross-linked polymer exhibits enhanced gas permeability to hydrogen, carbon dioxide, nitrogen, and methane as compared to the unmodified analog, without significant loss of selectivity, at temperatures from about 20 degrees Celsius to about 400 degrees Celsius. The cross linking agent has the general formula,
wherein R1 and R2 are independently selected from alkyl having from 1 to 20 carbons, aryl having from 6 to 18 carbons, substituted aryl; wherein R1 and R2 are connected to form a ring structure having from 2 in 5 carbons; and wherein X and Z are independently selected from chloride, bromide, and iodide The cross linked polymer is given below,

When ionic liquids (ILs) are impregnated in the porous membranes, all the pores of the porous support should be filled by IL. Moreover, only IL fraction is useful for the effective separation of gases.
In supported ionic liquid membranes (SILMs), liquid is held in the pores of the support via relatively weak capillary forces. When the transmembrane pressure differential is greater than those forces, the liquid will be pushed through the support, destroying the membrane. As a result, the SILMs mentioned above were only tested at pressure differentials of —0.2 atm (Ref: J. Membr. Sci. 2004, 238, 57; Ind. Eng. Chem. Res. 2007, 46, 5397-5404). Further, Poly (RTIL) films are reported to be brittle to make mechanically stable membranes.
Vinyl polymers suffer from more drawbacks such as: Vinyl containing monomers have limitations on structural architecture, they are porous and such porous polymers carry the threat of draining of liquid through the pores. As exemplified in the literature, present PFILs based on vinyl monomers are brittle and need to be crosslinked to form the film for membrane based separations.
Copolymers incorporating imidazole moiety such as polyether benzimidazoles, poly(arylene ether benzimidazole)s, poly(imide amide benzimidazole), poly(aryl ether benzimidazoles) and hyper branched polybenzimidazoles are known in literature. However, no prior arts have explored PFIL containing ionic liquid component incorporated in the imidazole polymer backbone itself, unlike vinyl polymers, which contain ionic liquid imidazole component in the side chain.
Therefore, there exists a need for polymeric forms of ionic liquids, where, the ionic liquids can be incorporated in the polymeric back bone itself to enhance the
structural strength of the polymer thereby enabling the polymer to withstand to any differential pressure and thus with enhanced performance.
Therefore, the present invention aims at fulfilling the existing need for polymeric forms of imidazoles or benzimidazoles where the ionic liquid moiety is present in the backbone of the polymer chain and exhibit a high degree of sorption for gases like CO2 at differential pressure. There also exists a need to provide a process of making PFIL that allows structural modification, which also the present invention aims to fulfill in the current invention.
Another unfulfilled need in the literature is the ability of the PFILs to possess film forming characteristics without further modification or processing such as crosslinking. None of the PFILs reported in the literature have film forming characteristics. The present invention provides PFILs with film forming characteristics that can be used to adopt the PFILs in desirable forms such as films, sheets, granules, flakes, powders and such like.
There is also a need in the art to provide a stable polymeric form of ionic liquid that possesses a high degree of gas sorption, permeation and separation capacity.